Exhaust device for a turbine engine

ABSTRACT

The invention relates to an exhaust device for a turbine engine, comprising a liner made of a fiber-reinforced ceramic matrix composite. The liner is preferably fitted some distance away from the inside of the exhaust section. The liner has a thickness of 1 to 10 mm, preferably of 2 to 6 mm. There is a gap of at least 10 mm, preferably 10 to 40 mm, between the liner and the inside of the exhaust.

BACKGROUND OF THE INVENTION

The invention relates to an exhaust device for a turbine engine.

Turbine engines, for example turbine engines for aircraft, produce ahigh noise intensity level, which is a quantity of sound which ismeasured in decibels (dB). A high noise intensity level not onlypollutes the environment but is also disadvantageous for the degree ofloading of aircraft. After all, the higher the degree of loading, thegreater the power which has to be supplied by the turbine engines, whichresults in a high and sometimes an unacceptable noise intensity level.

A reduction in the noise intensity level of turbine engines is effected,inter alia, by means of a special construction of the turbine engine.Thus, in modern turbine engines only a small portion of the incoming airstream flows through the combustion chambers. The major portion of theincoming air stream is diverted. This diverted air stream rejoins theexhaust gases in the exhaust section. Because, as a result of the highflow rate, the diverted air stream is essentially a turbulent airstream, this results in a reduction in the noise intensity level whichis caused by the outgoing air stream from the turbine engine.Nevertheless, a further reduction in the noise intensity level isdesirable.

The noise intensity level of turbine engines can also be reduced byfitting a noise-damping material, for example perfolin, in the inletsection of a jet engine. In this case the reduction in the noiseintensity level is achieved by so-called quench interference, that is tosay the sound waves which penetrate into the sound-damping materialextinguish one another by interference. However, these noise-dampingmaterials are not suitable for use in the exhaust section of turbineengines because they are not able to withstand the thermal andmechanical stresses which prevail in the exhaust section. Althoughnoise-damping materials are known which are able to withstand suchstresses, these materials have the disadvantage that they are too heavyfor use in aviation.

Although some ceramic materials, such as foams, are known which possessnoise-damping characteristics, monolithic, ceramic materials of thistype have the disadvantage that they have too low a resistance tofatigue and brittle fracture characteristics.

SUMMARY OF THE INVENTION

The aim of the invention is, therefore, to provide a material with whichthe noise intensity level of an engine, in particular of a turbineengine, can be reduced at the exhaust side of said engine. The inventiontherefore relates to an exhaust device for an engine which comprises aliner made of a fibre-reinforced ceramic matrix composite.

The engine can be an internal combustion engine, for example a petrolengine or a diesel engine, but also a turbine engine. The turbine enginecan be for stationary applications, for example a turbine engine or agas turbine for energy-generating installations, or a turbine engine foran aircraft, for example a turbo-propeller engine. The engine can alsobe a portable internal combustion engine, such as those used inhand-held power saws, motor mowers and the like.

The fibre-reinforced ceramic matrix composite has good noise-dampingproperties and is well able to withstand thermal and mechanicalstresses.

According to the invention it is preferable that the liner, whichcomprises the fibre-reinforced ceramic matrix composite, is enclosedwithin the exhaust device in such a way that the liner is fitted somedistance (d) away from the inside of the exhaust section of the engine.In this case no contact transfer of noise can occur and a maximumreduction in the noise intensity level is achieved.

Contact transfer of noise is understood to be transfer of sound wavesfrom one material to another material, the transfer taking place becausethe two materials are in contact with one another.

From the economic standpoint, and, in particular, to restrict theincrease in the weight of the engine, which is especially important inthe case of turbine engines for aircraft in connection with the usableloading capacity of the aircraft, it is advantageous to fit a thinliner. According to the invention, a liner which has a thickness of 1 to10 mm, in particular 2 to 6 mm, is preferably fitted.

According to the invention it is also preferable that there is a gap, inwhich air, exhaust gases and the like can be present or through whichthe latter can flow, between the liner, which comprises thefibre-reinforced ceramic matrix composite, and the inside of the exhaustsection. Said gap must be sufficiently large because otherwiseappreciable transfer of sound can take place by radiation of soundwaves, with the result that it is not possible to achieve optimumreduction of the noise intensity level. The gap between thefibre-reinforced ceramic matrix composite and the inside of the exhaustsection makes an important contribution to the reduction in the noiseintensity level irrespective of the transfer of sound through the wallof the exhaust section. According to a preferred embodiment of theinvention, the gap between the liner and the inside of the exhaustsection is at least 10 mm, preferably 10 to 40 mm.

The liner, which comprises the fibre-reinforced ceramic matrixcomposite, can be fitted as a whole. However, if the liner is very largein size or if it is desirable to fit a relatively thick liner, it ispreferable, according to the invention, to fit a liner which is made upof segments or sections of the fibre-reinforced ceramic matrix compositein the exhaust section. Another advantage of a liner consisting ofsections is that fitting thereof is simpler. Furthermore, local damageor worn spots in the liner can be repaired simply by replacing one ormore damaged or worn sections by new sections.

The sections made of the fibre-reinforced ceramic matrix composite canbe of circular, oval, rectangular or square shape. According to apreferred embodiment, the components are cylindrical. The componentscan, for example, be in the shape of a tile.

The invention also relates to a fibre-reinforced ceramic matrixcomposite for reducing the noise intensity level of turbine engines, inparticular of turbine engines for aircraft.

The composite comprises fibres, which are woven, knitted, stitched orcemented to one another, and a matrix of a ceramic material. In apreferred embodiment of the invention, the composite has a fibre contentof 20 to 60% by vol. and a matrix content of 10 to 30% by vol.

The composite according to the invention can be produced by using knownprocesses. Said processes can take place in the gas phase or liquidphase, optionally combined with a solid phase. An example of a gas phaseprocess is chemical vapour infiltration. Processes which take place inthe liquid phase are, for example, melt impregnation, sol-gelimpregnation and immersion techniques. A review of suitable processes isgiven, for example, in Y. G. Roman, “Toepassingen van keramischematrixcomposieten als functionele en structurele componenten”(“Applications of ceramic matrix composites as functional and structuralcomponents”), Materialen, 29 (1995), pp. 29-36.

The fibre-reinforced ceramic matrix composite according to the inventionmust be porous in order to achieve a substantial reduction in the noiseintensity level. The composite preferably has an open porosity of 40 to70% by vol., where the open porosity is the ratio of the open porevolume to the total composite volume, a gas permeability of 1⁻¹⁴ to 10⁻⁹m² and a tortuosity of 1 to 6. For this purpose the open porosity wasmeasured by means of helium pycnometry or by means of the Archimedesmethod (provisional standard NVN-ENV 1389 (1995)), the gas permeabilityby Darcy's method, which is described in R. E. Collins, “Flow of fluidsthrough porous materials”, Reinhold Publ. Corp., New York, Ed. R. Wilke,(1961), and the tortuosity by a method described by P. C. Carman, Trans.Inst. Chem. Eng. London, 15 (1937), p. 150.

The fibre-reinforced ceramic matrix composite according to the inventionhas a density of, preferably, less than 8 kg/dm³, which is advantageousfor applications in aviation.

The material from which the fibres and the matrix of the compositeaccording to the invention are produced is not of essential importancefor the sound-damping effect of the composite. The fibres can, forexample, be produced from carbon, oxides, carbides, nitrides, borides ormixtures thereof, said oxides, carbides, nitrides, borides or mixturesthereof containing magnesium, aluminium, silicon, the elements fromgroups III, IV, V, VI, VII and VIII or mixtures thereof. The matrix can,for example comprise any ceramic or carbon-containing material, such asmaterials which are produced from carbon, oxides, carbides, nitrides,borides or mixtures or compounds thereof, said oxides, carbides,nitrides, borides or mixtures thereof containing magnesium, aluminium,silicon, the elements of groups III, IV, V, VI, VII and VIII or mixturesor compounds thereof. Examples of such materials contained in the matrixare silicon carbide and aluminium oxide. According to a preferredembodiment of the invention, the fibres are carbon fibres and the matrixcomprises silicon carbide as the ceramic material.

The fibre-reinforced ceramic matrix composite can be protected againstcorrosion by, for example, oxygen, water or corrosive exhaust gases suchas hydrocarbons, carbon dioxide, sulphur dioxide and nitrogen oxides, bycoating the composite with a layer of a corrosion-resistant material.Preferably, the composite according to the invention is coated with oneor more layers or a multi-layer of carbon, silicon carbide, siliconnitride, boron nitride, aluminium oxide, silicon oxide, titaniumnitride, titanium diboride, yttrium oxide, zirconium oxide or mixturesor compounds thereof. The composite according to the invention is, inparticular, coated with one or more layers or a multi-layer of siliconcarbide.

The fibre-reinforced ceramic matrix composite according to the inventioncontains a layer which is located between the fibres and the matrix. Thepurpose of this layer is to provide the composite material with a“tough” or plastically deformable deformation characteristic. Theintermediate layer can be a layer of any suitable material, such as, forexample, carbon, boron nitride, metallic layers (platinum, silicon,tungsten and the like), intermetallic layers, for example suicides, oranother plastically deformable layer or layers or multi-layer. Saidlayers are about 0.1 to 1 micrometre thick; they can be porous and theyenvelop each individual fibre filament.

The fibre-reinforced ceramic matrix composite according to the inventioncan substantially reduce the noise intensity level within the frequencyrange from about 500 Hz to about 12 kHz. The greatest reduction takesplace at frequencies to which the human ear is most sensitive. Saidfrequencies comprise the range from about 500 Hz to about 8 kHz. Thegreatest reduction, that is to say more than 50%, in the noise intensitylevel is achieved in the frequency range from about 2 to 7 kHz. In thefrequency range from 3 to 5 kHz the reduction in the noise than 80%.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained with reference to, inter alia, adrawing, in which:

FIG. 1 shows a measurement set-up for measuring the absorptioncoefficient of the fibre-reinforced ceramic materials according to theinvention;

FIG. 2 shows a simplified substitute electrical circuit for the set-upaccording to FIG. 1;

FIG. 3 shows the absorption coefficient of various fibre-reinforcedceramic materials according to the invention as a function of thefrequency on the basis of simulations, where lines a, b, c and dindicate the absorption coefficient as a function of the frequency at,respectively, 10⁵, 10⁶, 10⁷ and 10⁸ N.s/m⁴.

DETAILED DESCRIPTION

In the possible measurement set-up according to FIG. 1, the referencenumeral 1 indicates a noise generator. The noise generator 1 generatesan electric noise signal, which is fed to a loudspeaker for conversioninto a sound signal. The sound signal, indicated here by arrow P2,propagates in a section 3 of a rectangular or circular pipe 4.

The section 3 is connected to the pipe at point 8. As desired, a calmair stream, indicated here by P1, can propagate in the pipe 4, which airstream is added to the sound signal P2 at connection point 8.

Two microphones M1, M2 for receiving sound wave P2, which is propagatingin pipe 4, are located downstream of connection point B. Microphones M1,M2 are connected to their signal processing unit 5 for processing thesignals received by the microphones M1, M2.

Further downstream from the microphones M1, M2 there is a test section6, in which the fibre-reinforced ceramic material 9 to be tested islocated. A closure 7 for the pipe 4 is located downstream of the testsection 6. If the abovementioned calm air stream 1 is used, the closure7 must be reflection-free, so that the calm air stream P1 is able toleave the pipe 4 without being reflected.

FIG. 2 shows a substitute electrical replacement circuit for the set-upaccording to FIG. 1. This circuit shows a power source U, whichcorresponds to the noise generator 1 and the sound source 2 in FIG. 1.The pipe 4 is represented by means of two leads with characteristicimpedance Z_(o), whilst the test section 6 from FIG. 1 is represented byan impedance Z_(t) which terminates the leads.

During a test, the installation according to FIG. 1 functions asfollows. The noise generator 1 generates a signal of arbitrary frequencycontent which is fed to the sound source 2, which converts this signalinto sound signal P2. The sound signal P2 propagates through the section3 towards the microphones M1, M2. With the aid of the set-up of twomicrophones M1, M2 arranged next to one another, it is possible todetermine not only the intensity of the sound signal P2 but also thepropagation direction, as will be clear to any acoustics expert. Thesignal processing unit 5 is used for processing.

The test section 6 now serves as a closure for pipe 4 with an impedanceZ_(t) which does not equal the characteristic impedance Z_(o) of thepipe 4, as a result of which a portion P_(r) of the sound signal P2 isreflected in a direction opposed to the direction of P2. The value ofthe impedance Z_(t) depends on the fibre-reinforced ceramic material 9chosen.

The intensity of the reflected sound signal P_(r) can be measured by themicrophones M1, M2 and, as a result of the use of two microphones M1, M2placed alongside one another, the reflected sound signal P_(r) can beseparated from the sound signal P2.

The ratio between the intensity of the reflected sound signal P_(r) andthe intensity of the sound signal P2 is now a measure for the impedanceZ_(t) of the test section 6 and thus a measure for the absorptioncoefficient of the chosen fibre-reinforced ceramic material 9, which islocated in the test section 6.

The reflection-free closure 7 prevents further reflections of the soundsignal P2 being produced at the end of the pipe 4, which furtherintensities would be able to influence adversely the intensity of thereflected sound signal P_(r).

FIG. 3 shows the dependence of the absorption coefficient of variousfibre-reinforced ceramic materials on the frequency. FIG. 3 showssimulation data for fibre-reinforced ceramic materials for fourdifferent permeability values (or air-resistance values) of thefibre-reinforced ceramic material. Here the permeability value isexpressed as (pressure drop over the sheet)/(gas speed×thickness of thesheet), in accordance with Darcy's law. The permeability value of thematerial can thus be changed as a function of, for example, thethickness of the sheet or the porosity of the material.

From FIG. 3 it can be seen, inter alia, that for a permeability value of10⁵ N.s/m⁴ (line a) an absorption coefficient of more than 80% can beexpected in a large portion of the frequency range of interest.

What is claimed is:
 1. In an engine having an exhaust section, anexhaust device connected to the exhaust section of the engine, theimprovement comprises a liner enclosed in the exhaust device a distancefrom the exhaust section of the engine, the liner being formed of afibre-reinforced ceramic matrix composite characterized by noise-dampingproperties and ability to withstand thermal and mechanical stresseswherein the distance (d) is sufficient to substantially eliminatecontact transfer of noise and thereby achieve a reduction in noiseintensity level.
 2. Exhaust device according to claim 1, wherein theliner has a thickness of 1 to 10mm.
 3. Exhaust device according to claim1, wherein the gap between the liner and the inside of the exhaust is atleast 10 mm.
 4. Exhaust device according to claim 1, wherein the linercomprises sections made of fibre-reinforced ceramic matrix composite. 5.Exhaust device according to claim 1, wherein the fibre-reinforcedceramic matrix composite has a fibre content of 20 to 60% by vol. and amatrix content of 10 to 30% by vol.
 6. Exhaust device according to claim5, wherein the fibre-reinforced ceramic matrix composite has an openporosity of 40 to 70% by vol., a gas permeability of 10⁻¹⁴ to 10⁻² m²and a tortuosity of 1 to
 6. 7. Exhaust device according to claim 5,wherein the fibre-reinforced ceramic matrix composite has a density ofless than 8 kg/dm³.
 8. Exhaust device according to claim 5, wherein thefibre-reinforced ceramic matrix composite has fibres which are carbonfibres and the ceramic matrix composite comprises silicon carbide. 9.Exhaust device according to claim 5, wherein the fibre-reinforcedceramic matrix composite is coated with a corrosion-resistant material.10. Exhaust device according to claim 5, wherein the fibre-reinforcedceramic matrix composite is coated with silicon carbide.
 11. Exhaustdevice according to claim 5, wherein the fibre-reinforced ceramic matrixcomposite has a layer located between the fibres and the matrix. 12.Exhaust device according to claim 1, wherein the liner has a thicknessof 2 to 6 mm.
 13. Exhaust device according to claim 1, wherein the gapbetween the liner and the inside of the exhaust is at least 10 to 40 lmm.